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From: Ming Lei <ming.lei@redhat.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
	syzkaller-bugs@googlegroups.com,
	syzbot <syzbot+61acc40a49a3e46e25ea@syzkaller.appspotmail.com>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: splice: infinite busy loop lockup bug
Date: Fri, 7 Aug 2020 21:41:14 +0800	[thread overview]
Message-ID: <20200807134114.GA2114050@T590> (raw)
In-Reply-To: <20200807123854.GS1236603@ZenIV.linux.org.uk>

On Fri, Aug 07, 2020 at 01:38:54PM +0100, Al Viro wrote:
> On Fri, Aug 07, 2020 at 01:27:27PM +0100, Al Viro wrote:
> > On Fri, Aug 07, 2020 at 07:35:08PM +0900, Tetsuo Handa wrote:
> > > syzbot is reporting hung task at pipe_release() [1], for for_each_bvec() from
> > > iterate_bvec() from iterate_all_kinds() from iov_iter_alignment() from
> > > ext4_unaligned_io() from ext4_dio_write_iter() from ext4_file_write_iter() from
> > > call_write_iter() from do_iter_readv_writev() from do_iter_write() from
> > > vfs_iter_write() from iter_file_splice_write() falls into infinite busy loop
> > > with pipe->mutex held.
> > > 
> > > The reason of falling into infinite busy loop is that iter_file_splice_write()
> > > for some reason generates "struct bio_vec" entry with .bv_len=0 and .bv_offset=0
> > > while for_each_bvec() cannot handle .bv_len == 0.
> > 
> > broken in 1bdc76aea115 "iov_iter: use bvec iterator to implement iterate_bvec()",
> > unless I'm misreading it...
> > 
> > Zero-length segments are not disallowed; it's not all that hard to filter them
> > out in iter_file_splice_write(), but the intent had always been to have
> > iterate_all_kinds() et.al. able to cope with those.
> > 
> > How are these pipe_buffers with ->len == 0 generated in that reproducer, BTW?
> > There might be something else fishy going on...
> 
> FWIW, my preference would be to have for_each_bvec() advance past zero-length
> segments; I'll need to go through its uses elsewhere in the tree first, though
> (after I grab some sleep),

Usually block layer doesn't allow/support zero bvec, however we can make
for_each_bvec() to support it only.

Tetsuo, can you try the following patch?

diff --git a/include/linux/bvec.h b/include/linux/bvec.h
index ac0c7299d5b8..b03c793dd28d 100644
--- a/include/linux/bvec.h
+++ b/include/linux/bvec.h
@@ -117,11 +117,19 @@ static inline bool bvec_iter_advance(const struct bio_vec *bv,
 	return true;
 }
 
+static inline void bvec_iter_skip_zero_vec(const struct bio_vec *bv,
+		struct bvec_iter *iter)
+{
+	iter->bi_idx++;
+	iter->bi_bvec_done = 0;
+}
+
 #define for_each_bvec(bvl, bio_vec, iter, start)			\
 	for (iter = (start);						\
 	     (iter).bi_size &&						\
-		((bvl = bvec_iter_bvec((bio_vec), (iter))), 1);	\
-	     bvec_iter_advance((bio_vec), &(iter), (bvl).bv_len))
+		((bvl = bvec_iter_bvec((bio_vec), (iter))), 1);		\
+	  (bvl).bv_len ? bvec_iter_advance((bio_vec), &(iter), (bvl).bv_len) : \
+			bvec_iter_skip_zero_vec((bio_vec), &(iter)))
 
 /* for iterating one bio from start to end */
 #define BVEC_ITER_ALL_INIT (struct bvec_iter)				\

Thanks,
Ming


  reply	other threads:[~2020-08-07 13:43 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-02 18:26 INFO: task hung in pipe_release (2) syzbot
2020-08-07 10:35 ` splice: infinite busy loop lockup bug Tetsuo Handa
2020-08-07 12:27   ` Al Viro
2020-08-07 12:34     ` Tetsuo Handa
2020-09-01  0:51       ` Qian Cai
2020-09-01  1:09         ` Al Viro
2020-09-01  1:47           ` Qian Cai
2020-09-01  3:32           ` Qian Cai
2020-09-01  4:07             ` Al Viro
2020-09-01  1:10         ` Ming Lei
2020-09-01 14:22           ` Qian Cai
2020-08-07 12:38     ` Al Viro
2020-08-07 13:41       ` Ming Lei [this message]
2020-08-07 14:11         ` Matthew Wilcox
2020-08-07 15:11           ` Tetsuo Handa
2020-08-09  2:31           ` Ming Lei
2020-08-09  2:49             ` Ming Lei
2020-08-07 14:17         ` Tetsuo Handa
2020-08-13  3:57 ` INFO: task hung in pipe_release (2) syzbot

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